Over the past 20 years, there has been an evolution in the thermal processes applied for recovering heavy, viscous oil from subterranean reservoirs in Alberta.
The first commercially applied process was cyclic steam stimulation. This process is commonly referred to as "huff and puff". Steam is injected into the formation, commonly at above fracture pressure, through a usually vertical well for a period of time. The well is then shut in for several months, referred to as the "soak" period. Then the well is opened to produce heated oil and steam condensate until the production rate declines. The entire cycle is then repeated. In the course of the process, an expanding "steam chamber" is gradually developed. Oil has drained from the void spaces of the chamber, been produced through the well during the production phase, and is replaced with steam. Newly injected steam moves through the void spaces of the hot chamber to its boundary, to supply heat to the cold oil at the boundary.
There are problems associated with the cyclic process. More particularly:
The fracturing tends to occur vertically along a direction dictated by the tectonic regime present in the formation. In the Cold Lake area of Alberta, fracturing tends to occur along a north-east trend; PA1 When steam is injected, it tends to preferentially move through the fractures and heat outwardly therefrom. As a result, the heated steam chamber that is developed tends to be relatively narrow and extends along this north-east direction from opposite sides of the well; PA1 Therefore large bodies of unheated oil are left in the zone extending between adjacent wells and their linearly extending steam chambers; and PA1 The process is not efficient with respect to steam utilization. PA1 Providing a pair of coextensive horizontal wells spaced one above the other. The spacing of the wells is typically 5-8 meters. The pair of wells is located close to the base of the formation; PA1 The span of formation between the wells is heated to mobilize the oil contained therein. This may be done by circulating steam through each of the wells at the same time to create a pair of "hot fingers". The span is slowly heated by conductance; PA1 When the oil in the span is sufficiently heated so that it may be displaced or driven from one well to the other, fluid communication between the wells has been established and steam circulation through the wells is terminated; PA1 Steam injection at less than formation fracture pressure is now initiated through the upper well and the lower well is opened to produce draining liquid. Injected steam displaces the oil in the inter well span to the production well. The appearance of steam at the production well indicates that fluid communication between the wells is now complete; PA1 Steam-assisted gravity drainage recovery is now initiated. Steam is injected through the upper well at less than fracture pressure. The production well is throttled to maintain steam trap conditions. That is, throttling is used to keep the temperature of the produced liquid at about 6-10.degree. C. below the saturation steam temperature at the production well. This ensures that a short column of liquid is maintained over the production well, thereby preventing steam from short-circuiting into the production well. As the steam is injected, it rises and contacts cold oil immediately above the upper injection well. The steam gives up heat and condenses; the oil absorbs heat and becomes mobile as its viscosity is reduced. The condensate and heated oil drain downwardly under the influence of gravity, The heat exchange occurs at the surface of an upwardly enlarging steam chamber extending up from the wells. The chamber is fancifully depicted in FIG. 1. The chamber is constituted of depleted, porous, permeable sand from which the oil has largely drained and been replaced by steam. PA1 The steam chamber continues to expand upwardly and laterally until it contacts the overlying impermeable overburden. The steam chamber has an essentially triangular cross-section. If two laterally spaced pairs of wells undergoing SAGD are provided, their steam chambers grow laterally until they contact high in the reservoir. At this stage, further steam injection may be terminated and production declines until the wells are abandoned. PA1 There is a need to more quickly heat the formation laterally between laterally spaced wells; and PA1 As previously stated and as illustrated in FIG. 1, the steam chambers produced by pairs of SAGD wells are generally triangular in cross-section configuration. As a result there is unheated and unrecovered oil left between the chambers in the lower reaches of the reservoir (this is indicated by cross-hatching in FIG. 1). PA1 (a) providing a pair of spaced apart, generally parallel and co-extensive, generally horizontal steam injection and production wells; PA1 (b) establishing fluid communication between the wells; PA1 (c) practising steam-assisted gravity drainage to recover oil by injecting steam at less than formation fracture pressure (typically at a low pressure that is greater than but close to formation pressure) through the injection well and producing steam condensate and heated oil through the production well while throttling the production well as required to keep the produced liquid temperature less than the steam saturation temperature at the injection well (that is, operating the production well under steam trap control); PA1 (d) providing a horizontal third well, generally parallel and co-extensive with the injection and production wells and preferably located at about the same general elevation as the pair of wells, the third well being laterally offset from the pair of wells, typically at a distance of about 50 to 80 m; and PA1 (e) contemporaneously practising cyclic steam stimulation at the offset well, preferably by injecting steam at less than formation fracture pressure, more preferably at a "high" pressure which is greater than that being used at the SAGD pair, and preferably by operating the well during the production phase under steam-trap control conditions, to develop a steam chamber which causes lateral heating of the span of reservoir formation between the pair of wells and the third well and to periodically produce heated oil through the offset well. PA1 if SAGD and huff and puff are practised contemporaneously using horizontal wells at laterally spaced locations; and PA1 if the huff and puff well is converted to fluid production under steam trap control when fluid communication has been established between the locations;
Steam/oil ratios are relatively high because the steam is free to be driven down any permeable path.
In summary then, huff and puff gives relatively low oil recovery and the steam/oil ratio is relatively high.
A more recent, successfully demonstrated process involves a mechanism known as steam-assisted gravity drainage ("SAGD").
One embodiment of the SAGD process is described in Canadian patent 1,304,287. This embodiment involves:
The SAGD process is characterized by several advantages, relative to huff and puff. Firstly, it is a process involving relatively low pressure injection so that fracturing is not likely to occur. The injected steam simply rises from the injection point and does not readily move off through fractures and permeable streaks, away from the zone to be heated. Otherwise stated, the steam tends to remain localized over the injection well in the SAGD process. Secondly, steam trap control minimizes short-circuiting of steam into the production well. And lastly, the SAGD steam chambers are broader than those developed by fracturing and huff and puff, with the result that oil recovery is generally better. It has been demonstrated the better steamloil ratio and oil recovery can be achieved using the SAGD process.
However there are a number of problems associated with the SAGD process which need addressing. More particularly:
It is the objective of the present invention to provide a SAGD process which is improved with respect to these shortcomings.